Merged image of a V8 engine, Sherman Tank,  Clementine the Cat, and my MGA in the Alpes

MGA

Restoration

MGA restoration

Road trips

To the Alps in an MGA

MGA to Morzine 2008

Engine and cooling

MGA cooling

Fuel vaporisation

Hot starting

Interior

MGA seats - making them comfortable

Electrical

Alternator and negative earth conversion

MGA Fuel Vaporisation

www.VORD.net


The MGA has conked out in traffic a few times due to fuel vaporization. The only way to cure the problem was to let the car cool down with the bonnet open for half an hour, then she'd start and run perfectly.

Yesterday she broke down just at the top of a ramp in a multi storey car park. The fuel vaporisation needed some attention.

Original MGA heatshield

The MGA engine bay has it's issues. Not least the carburetor float chambers sit only an inch from the exhaust manifold, and the steel heat shield is painted in a dark colour which is poor for reflecting heat. Also the engine bay is very enclosed so air flow through the engine bay is poor and heat can build up.

My plan was to replace the old steel heatshield with a shiny aluminium one that might reflect a bit of heat.

   
Heatshield removed

You can see the proximity of the float chamber to the exhaust with the heatshield removed.

Modern car practice is to have heatshields right up against the exhaust manifolds, the idea being that heat is taken down the exhaust and released under the floor rather than in the engine bay.

But enclosing the exhaust manifold can increase the temperature towards the ignition point for oil and petrol, so wouldn't necessarily be a sensible option for a car with a track record of leaking both of these fluids around the engine bay.

   
New aluminium heatshield

I made the new heatshield slightly larger than the old one, and positioned it slightly closer to the exhaust.

The old heat shield made a good template for the hole positions which were drilled after all the folding and bending had been finished. I've also included an adjustable throttle linkage much like the one my bicycle brakes used to have.

   
Carburettor float chamber heatshields

I quite enjoyed making the heatshield so decided to knock a couple more up. The cunning idea was to fit the main heatshield on the engine side of the carburetor insulating blocks, and these further heatshields on the carb side to shield the float chambers.

   
Heatshields installed

Here the heatshields are installed, and I thought I'd test them out with a thermocouple.

After an enthusuiastic 5 mile drive and a long idle I opened up the bonnet. The heatshield closest to the exhaust was quite hot to the touch. The second heatshield was much cooler, and a the float chambers were 5 degrees C cooler than that.

Success - however well the new main heatshield is performing, the secondary ones are reducing fuel temperatures by 5 degrees C. That's got to help.

   
Thermolcouple location

 

I thought I'd do some measurements. Here's a photo of the thermocouple installation. It's a cheap one that came with the volt meter that's taped to the wing. I could see the readings while driving.

The outside temperature was about 15 degrees C, and the float chamber stayed at around 35 degrees C when driving on the open road. After some stop start city driving the float chamber temperature slowly increased. At 45 degrees C the idle speed started to fall a little. The temperature rose to 49 degrees before I got bored and decided to investigate.

Turns out everything in the engine bay was at that temperature, and of course it would be - there's a great big radiator at the front heating everything up.

   
electric cooling fan

I fitted a manual switch to the engine cooling fan to see whether a constant airflow would help, but it didn't make any difference to the fuel temperature. I did wire the fan from live rather than ignition so the fan can cool the engine down while I'm buying my groceries, hopefully getting around the embarrassing hot starting problems.

The one thing that did help was a little fan from a computer directed from the cold air vents to the float chambers. That's my emergency backup plan, but I'll keep the car as it is for now, as I suspect the heatshields were the cause of the problem. Although I really wish I'd measured float chamber temperatures before I started.

The float chambers must have been reaching the boiling point of petrol (around 100 degrees C), so if I'm only seeing 50 degrees now then I've hopefully cured the problem.

Update: The changes to the heatshield have eliminated fuel vaporisation as a problem, but I later developed a hot running problem, mostly down to ignition leads and rotor arm.

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